Heat oven to 350° with rack set in lower-middle position.

Cake

Whisk together 1 tablespoon cocoa powder and 1 tablespoon melted butter to make a paste. Thoroughly coat the interior of a 12-cup bundt pan with cocoa-butter paste using a pastry brush.

Combine remaining ¾ cup cocoa powder and chocolate in a medium bowl. Microwave coffee in a glass measuring cup until simmering, then pour over chocolate mixture, cover, and let stand 5 minutes. Whisk chocolate mixture until smooth, let cool slightly, then whisk in sour cream.

In a separate bowl, whisk together flour, salt, ground ginger (if using), and baking soda.

In a stand mixer fitted with the paddle attachment, beat butter, sugar, and vanilla on medium-high speed until pale and fluffy, about 4 minutes.

Add eggs, one at a time, beating after each addition. Reduce speed to low and add flour in three additions, alternating with chocolate-sour cream mixture, ending with flour, scraping down sides of bowl as needed.

Add candied ginger (if using) and mix just until incorporated.

Transfer batter to prepared pan and smooth top with a spatula. Bake cake until a skewer inserted in the center comes out with a few crumbs, 45–50 minutes.

Let cake cool in pan 10 minutes, then invert onto a wire rack and cool completely.

Glaze

Place chocolate in a medium bowl. Bring cream to a simmer in a small saucepan then pour over chocolate and let stand 2 minutes. Add butter and whisk until combined, let cool slightly.

Drizzle glaze over cake and sprinkle candied ginger (if using); let glaze set before serving.